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PEOCEEDINGS OF THE NATIONAL MUSEUM 



VOL. 68 



X (102) remaining the same. The latter orientation makes a (100) 

 prominent as in the previously described crystals, while the stria- 

 tions on e (Tl2) become normal in direction and Y (324) becomes 

 X (Tl3). The assumption that the latter is the correct orientation 

 would make it evident that the crystals were measured in the Gold- 

 schmidt, rather than the Dana, orientation. The aiigles measured 

 may .therefore be compared with those in the Winkeltabellen and 

 the forms thus identified may be transposed to those for the Dana 

 position. The angles are below compared with those for the Gold- 

 schmidt position and the equivalent indices for the Dana orienta- 

 tion are given in the last column. 



Comparison of angles measured on datolite crystal, Figure 5, with angles for 

 Goldschmidt position and equivalent indices for the ttvo positions 



The above table shows that the alternate position gives no closer 

 agreement iii angles and, although the face which is a new form 

 (2.3.10) in the first orientation becomes (534) an established form, 

 two others which in the first orientation are v (111) and e (211) estab- 

 lished forms with simple indices, become new forms with the more 

 complex indices (553) and (952) respectively. For these reasons the 

 orientation as drawn is believed to bo correct. The cause for the 

 abrupt departure of the crystals of this specimen from the habit 

 characteristic for the locality is not apparent. 



APOPHYLLITE 



Apophyllite is rare at Leesburg, occurring as scattered minute 

 colorless transparent crystals, seldom 1 mm. long, resting upon 

 crusts of datolite crystals. On other specimens the apophyllite is 

 largely altered and is opaque white, and friable, many of the crystals 

 being mere skeletons or shells. 



